A quantitative method for measuring antipsoriatic activity of drugs by the mouse tail test.

Topical treatment of the mouse tail with antipsoriatic drugs enhances orthokeratotic cell differentiation in the epidermal scales. We developed a new evaluation system for this test which allows quantification of drug efficacy. Drugs were applied topically, once daily, 5 times a week, for 2 weeks. Two hours after the last treatment the animals were sacrificed, longitudinal sections of the tail skin were made and prepared for histological examination (hematoxylin-eosin staining). As indicator of orthokeratosis (OK), the length of the granular layer per scale was measured microscopically with a semiautomatic image evaluation unit and related to the total scale length (= % OK per scale). Drug activity was defined by the increase in the total length of orthokeratotic regions, 100% activity corresponds to a granular layer extending over the whole scale length. In this model dithranol and retinoic acid dose-dependently increased orthokeratosis up to 75 and 79%, respectively. Beech tar, in a concentration of 5% induced a 19% increase in orthokeratosis. Morphometric quantification by image analysis of the conversion of parakeratotic into orthokeratotic regions in mouse tail scales induced by topical drug treatment seems to be a suitable and reliable procedure to investigate new drugs from which antipsoriatic activity may be expected.